The invention relates to hot-air devices with a heating element situated in a stream of air, with a carrier consisting of temperature-resistant, insulating material, which exhibits heating wires located in the air channels, and with at least one metal mount for the carrier to fix the carrier into the heating element.
Such hot-air devices are commercially available and known as hot-air appliances, modules or systems. The carrier situated in the air or gas stream usually consists of ceramics or another suitable material, and is accommodated in a heating tube, at whose one end air or gas is blown in. These hot-air devices have various modes for supplying air or gas streams. There are devices in which a fan is housed directly in the casing, and devices with external air stream generators. Various nozzles an typically be connected at the other end of the heating tube. The heating tube accommodates the heating element with the carrier made of temperature-resistant material, usually ceramic material, e.g., which is held by a central pin on a connection head arranged on the side facing the fan. This connection head is also used to electrically hook up the heating wires located in the carrier. Generally located on the side facing the nozzle there is an additional as well as a thin ceramic disk, which also essentially exhibits air channels, wherein this protective disk is not heated by the heating wires. The protective disk is moveable and separated from the carrier, but does also sit on the pin, whose free end is pinched, for example, to prevent the carrier and protective disk from sliding off.
DE 198 39 044 A1 discloses such a device, in which the heating wires are spirally arranged in air channels. In other known devices, the heating wires run in axially parallel air channels.
However, one problem is encountered in cases where the heating element is operated even though the air stream has dropped below minimal levels, e.g., due to fan failure or narrowing of the air inflow or outflow (nozzles). The heating element is here usually destroyed as a result. For this reason, the commercially available hot-air devices exhibit an additional sensor, e.g., thermocouples, optical sensors, resistance meters, etc., to prevent this undesired overheating.
CH 537 687 discloses a device on an electrical resistance heater to limit the heating element temperature to a specific value, in which the tubular heater encompasses a resistance wire that is embedded in a compacted, powdery insulating material. A metal tubular jacket envelops the insulating material with the wire, and the ends of the tubular jacket are tightly sealed by insulating plugs. When a certain maximal temperature is exceeded, the leakage current between the resistance wire and the tubular jacket due to the negative resistance characteristic of the powdery magnesium oxide is used to interrupt the heating circuit.
The object of this invention is to propose an improved hot-air device that achieves an effective heating element protection without additional sensors, e.g., thermocouples, optical sensors, resistance meters, etc., and additionally expands temperature control capability.
According to the invention, the carrier establishes partial, preferably complete, electrical contact with the metal mount. This mount is connected with an electrical circuit in order to control and/or regulate the heating circuit by means of a current that depends on the heating of the carrier and flows over the temperature-resistant, insulating material between the spiral heating wire situated in the air channels and the metal mount. The electrical circuit can disconnect the heating circuit when a specific measured variable is exceeded and/or reduce power. The measured variable can either be the leakage current flowing over the insulating ceramic material due to heating, or the insulation resistance determined by impressing a current onto a ceramic material. Also known from the CH publication is that the current flowing through the temperature-resistant material given a rising temperature can be acquired and used to control temperature. The heating wire is there embedded in the insulating material, so that a direct contact is established between the heating wire and insulating material. Surprisingly, it was found that this principle also leads to reproducible measuring results in existing hot-air devices, in which the heating wires are arranged in an air stream, i.e., not embedded in the material, so that the heating circuit can be controlled and/or regulated. The carrier is here either contacted on the outer periphery or by means of one or more longitudinal boreholes. To this end, it is important that a sufficient and good contact be achieved between the metal mount and the carrier material. The contact surface can here be the entire surface, or only a portion of the contactable surface in varying temperature zones of the carrier. This depends on which maximal current intensity is required and desired for further signal processingxe2x80x94temperature control/regulation. The lower the contact resistance between the heating wires and contactable surface, the higher levels to which the current can build up. The contact resistance of the heating wires on the ceramic carrier, the internal resistance of the carrier and the contact resistance of the carrier on the contact element are relevant for determining this current. At higher temperatures, the air in the respective air channel becomes ionized. This enables an addition current to flow between the heating wire and carrier, which influences the entire current through the ceramic material. This current through the ceramic insulation material determined by the temperature-dependent insulation resistance and applied voltage between the heating wire and metal mount, will be defined as the measuring current below.
In a preferred design, the measuring current is derived via an already present mount, in particular the central mount. For example, this can be done by using a hollow tube as the mount, preferably polygonal tube, which is expanded from inside after placement of the carrier for purposes of adjusting to the ever-present tolerances of the temperature-resistant material and sufficiently contacting the latter. Expansion from inside can here take place mechanically, pneumatically or hydraulically. Another possibility would be to use round or cornered spring sleeves, which are placed on a metal rod, and hence realize a sufficient contact between the mount and the temperature-resistant material. The mount in the form of a metal pin or hollow tube can also be established through casting with a contacting material, e.g., liquid ceramic, in the gap between the periphery of the central mount and the channel in the carrier.
If not absolutely necessary for contacting the central mount with the carrier, either a hollow pin or a massive pin can be used. In a preferred embodiment, a hollow pin is used, and the temperature sensor is routed through the latter to the tip of the carrier. This makes it easy and inexpensive to additionally incorporate the required temperature acquisition process to regulate the heating wires at the point where the air exits the carrier. The jacket of the temperature sensor advantageously contacts the carrier or hollow tube, and derives the measuring current via the latter.
In a particularly preferred design, the carrier as contact surface exhibits at least one electrically conductive surface. To this end, the carrier is coated with a conductive material, e.g., silver, or another material that withstands the temperatures encountered for hot-air devices. The coated surface can here be arranged on the outer periphery or inside a borehole, into which a contact element (tube or temperature sensor) is introduced and, as already explained above, dimensioned and situated on the carrier in such a way that the measuring current is no longer acquired point by point, and hence is subject to strong fluctuations, but rather the integral is acquired and derived over all currents impacting the conductive surface.
In an advantageous embodiment of the invention, the electrical circuit controls and/or regulates the heating circuit as a function of the measured insulation resistance of the temperature-resistant insulating material. To measure the insulation resistance, the heating wire is separated from the power supply, and a measuring current is impacted via the metal mount and heating wire connection. The insulation resistance measured as a result is subsequently used for regulating and/or controlling the heating circuit. To be able to perform measurements during operation, it is necessary to this end that the electrical circuit interrupt the heating circuit at the time of measurement, and then activate it again later.